A fuel system in an aircraft is designed to supply high pressure fuel to the engines which power the aircraft. In addition, the high pressure fuel system is often utilized as a source of high pressure fluid for the hydraulic systems and servos which position actuators which control the engine or other aspects of the aircraft. Fuel systems must be sized to supply enough fuel to the engine to start it reliably. Most fuel pumps are sized at windmill start conditions, which are typically about 5–6% of normal speed. Gear pumps typically have low efficiency at these low speeds so the resulting pump size may be large to meet engine flow demands, which may include burn flow and servo flow to various actuators and air handling valves. The flow for the actuators and air handling valves may exceed that required by the engine. Hence, the resulting pump size from this sizing requirement can be quite large and can generate significant heat during normal engine operation. This heat, in conjunction with engine oil heat dissipation into the fuel system, may result in high fuel system temperatures, which in turn may result in low Hydro Mechanical Unit (HMU) seal life and lower Electro Mechanical Interface Device (EMID) life.
Some of the servo flow during an engine start or windmill start may be diverted to devices that may not need to be active during that portion of the engine start cycle. For example, the high pressure turbine active clearance control (HPTACC) and low pressure turbine active clearance control (LPTACC) valves may not be required during that portion, or other portions, of the engine start. Prior art systems have used electric solenoid valves to turn flow on and off to the non-required servos. However, this can be relatively expensive, heavy, and can consume significant electrical power
Accordingly, it would be desirable to lock out, or at least limit, flow to various devices that may not be needed during start (or portions thereof) with a pressure activated servo lockout valve to eliminate the weight and expense of a electric solenoid valve, which would allow a smaller main fuel pump size, and subsequently, less fuel-pump-generated heat during normal engine operation. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.